LHC Detectors 101

1. LHC Detectors 101 Vivek Sharma (with slides stolen from talks of several people ) Good review article: ARNPS 2006, “General purpose detectors for large Hadron collider” by Froidevaux & Sphicas

2. Cross Sections in pp Collision • Three components • pppp (elastic) • pppX (diffractive) • ppX (inelastic) • Don’t know to ~10% • Is ~100 mb • Will measure it (TOTEM) • Interaction rate: • N = Lx  • For L = 1033 cm-2 sec-1 : N = 100 MHz ! The good stuff is here and below Detectors need to keep All of the good stuff and reject most of the soft stuff

3. Detectors 101 • Detectors for high PT at colliders are designed to identify and measure the "objects" that are used to do physics • electrons • muons • taus • photons • quark and gluons as jets • including b-quark jets • neutrinos (and dark matter, etc) • as missing energy • A "generic" detector is a cylinder (with endplugs) with concentric layers of detector elements New physics always couples to electrons, muons and photons ..Samual CC Ting

4. look at interaction of different particles with the same high energy (here 300 GeV) in a big block of iron: 1m The energetic electron radiates photons which convert to electron-positron pairs which again radiate photons which ... This is the electromagnetic shower. electron The energetic muon causes mostly just the ionization ... muon pion (or another hadron) Electrons and pions with their “children” are almost comple- tely absorbed in the sufficiently large iron block. The strongly interacting pion collides with an iron nucleus, creates several new particles which interact again with iron nuclei, create some new particles ... This is the hadronic shower. You can also see some muons from hadronic decays.

5. Hadronic and Electromagmnetic Interaction with Material ! What about interactions of high energy photons? What about neutral pions which decay very quickly (the mean lifetime is just 8×10-17 s, ct = 25 nm) to two photons? To answer these questions think about the evolution of the electromagnetic cascade ... For a deeper insight to the electromagnetic and hadronic showers remember the exponential probability of a projectile to survive without interaction or without absorption in the depth t of the target: t, the mean interaction length determines the mean distance between collisions of hadrons with nuclei of the material and therefore it tells us where the hadronic shower will probably start and how fast it will evolve. The radiation lengthXhas almost the same meaning in evolution of the electromagnetic cascade – it determines the mean path of an electron to radiate the photon and also the mean path of a photon to convert to the electron-positron pair. Look at values of these quantities for several materials:

6. General strategy of a LHC detector to catch almost all particles Magnetic field bends the tracks and helps to measure the momenta of particles. electron Hadronic calorimeter: offers a material for hadronic shower and measures the deposited energy. muon Neutrinos escape without detection hadrons Muon detector: does not care about muon absorption and recordsmuon tracks. Tracker: Not much material, finely segmented detectors measure precise positions of points on tracks. Electromagnetic calorimeter: offers a material for electro- magnetic shower and measures the deposited energy.

7. Concentric Layers of Specialized Instrumentation muon hadronic calorimeter EM cal. Tracking in Solenoidal B-field to measure PT e tracking   , K, p.. 

8. A Slice of The CMS Pie

9. CMS: Exploded View Endcap Endcap Barrel

10. And what it actually looks Like

11. Physics Objects • Go through the physics objects one-by-one • Not many details, but general picture • How are they detected? • How well are they measured? • How are they misidentified? • Will conclude with score card on objects • A little understanding of how this works is useful to • Appreciate the forthcoming exp. results • If you want to participate in the LHC program

12. Aftermath of Collision: Charged particle in a Magnetic field leave their footprint in the Inner detector (Tracker) Simulated pp collisions

13. Electron signature • Track in the inner detector • Shower and complete energy deposition in EM calorimeter • electron bremsstrahlung • e+e- pair production http://www.irs.inms.nrc.ca/EGSnrc/pirs701/img12.png http://www.irs.inms.nrc.ca/EGSnrc/pirs701/node22.html

14. www-zeus.physik.uni-bonn.de/~brock/ Electron Signature (2) http://student.physik.uni-mainz.de/~reiffert/atlas/em-shower.jpg X0 = radiation length PbW04: 0.9 cm, Pb: 0.6 cm, Cu:1.4 cm

15. Photons • Just like electron, but no track • Resolution of EM calorimeters very good, eg, CMS • Gets better with increasing E • Question: where do all these terms come from? • Answers • 1st term: shower statistics (fluctuations of number of particles in shower) • 2nd term: mostly module-to-module calibration • 3rd term: noise, pileup, etc (E in GeV)

16. Hadrons • Track in inner detector (unless neutral, eg, n) • Hadronic interaction • Some energy deposition in EM calorimeter • Energy deposition in HAD calorimeter Interaction length  PbWO4: 22 cm Pb: 17 cm Cu: 15 cm http://student.physik.uni-mainz.de/~reiffert/atlas/hardron-shower.jpg

17. EM vs HAD showers • The pretty pictures look similar, but the physics is different • with important consequences • X0 <<  • Longitudinal (and transverse) evolutions quite different • e/ on average shower first and stop first • use it to separate e from  This is a good thing • Hadronic shower fluctuations large • Energy resolution poor • Response often not linear with E This is not a good thing

18. Calorimeter response to  (CMS) Non-linear response Poor resolution compared to e/

19. Jets • Traditionally reconstructed by summing the energy in nearby calorimeter towers • Limitations in the hadronic energy measurement leads to poor resolution

20. Muons • Measured in the inner tracker, go through the calorimeter, measured again outside CMS • Unlike electron case, resolution gets worse at high energy. Why?

21. Neutrinos (or dark matter) • Sum up the momenta of everything, what is left to get back to zero (missing energy) is the neutrino(s) • Longitudinal information is lost down the beam pipe  can only do in transverse plane • Missing transverse energy (MET) • If > 1 , you only infer the sum of the  transverse momenta

22. 1 min bias event contribution to MET component in a given direction  ~ 6 GeV CMS Missing Transverse Energy • Fake MET mostly from jets, resolutions and tails • Also from missed muons • Also from "underlying event" CMS And the tails don't come without some work.... D0